This is a great idea, if it doesn't terribly break the web. Maybe we should try it out too?

/cc Chris who is making this change in WebKit.

This sounds great for security to me! Is it possible we could easily add a UseCounter for accessing the opener from such windows (either before or after making this change)? I know some sites enumerate the whole global object, so maybe it's not useful to monitor for calls to the window.opener getter. 

Regardless, trying this out in Chrome too will get us additional data on the compat risk that Chris may not be able to get from Safari TP alone. FWIW, even if there's a little site breakage, I suspect this change would be worth making - we'll just need to follow the blink breaking change process...